Germ Cells from Pluripotent Stem Cells

September 30, 2013

Katsuhiko Hayashi and Mitinori Saitou of Kyoto University in Japan have successfully produced oocytes from induced pluripotent stem cells (iPSCs) and embryonic stem cells (ESCs) in mice (they had previously made sperm cells from the same materials). Importantly, though these experiments used mice models, the procedure presents the possibility–as several news sources have recently reported–of one day serving as a reproductive alternative for humans.

How Did They Do It?

Despite being used only on mice so far, this kind of technology raises several issues from a bioethics standpoint. In order to discern these issues, we need to understand a couple of key factors in the techniques, which are laid out in a Natureresearch paper and summarized in a Nature Newsarticle.

The egg cell (oocyte) is produced from an iPSC and/or ESC. While the particular cell type may seem like a trivial matter when dealing with mouse models, it could become an important distinction if this were to be done in humans, as it would involve the destruction of one embryo in order to make another.

Sperm cells still come only from male ESCs/iPSCs, while oocytes are formed only from female ESCs/iPSCs. The Nature News article reports that Hayashi and Saitou have been contacted by several organizations, including some hoping to find a way to produce egg and sperm from same-sex couples. In reality, while male iPSCs have been used to produce oocytes, it is an inefficient process, so the authors of this study chose to use female iPSCs to produce oocytes. Female iPSCs cannot yet yield sperm; this is due to the difficulties in converting a cell with XX (female) chromosomes to XY (male).*

The iPSCs or ESCs are converted into primordial germ cells, the cells that eventually develop into oocytes and sperm. They are notmade directly into the gametes themselves. A key step in this procedure involves the primordial germ cells being implanted back into male and female mice to become sperm and eggs there, because “the details of how [primordial germ cells] undergo gametogenesis in the transplanted tissue are unclear.” In other words, the researchers were unsure how primordial germ cells actually become their respective gametes, so they implanted them into live mice and let Nature provide the signaling mechanisms.

The subsequently developed gametes are removed and fertilized in vitro. Healthy embryos are selected and, finally, implanted into a surrogate mouse. To ensure that the offspring were from the original donor stem cells, Hayashi and Saitou implanted embryos from colored mice into albino mouse wombs. If the offspring were colored, then they clearly came from the stem cell source, not from the surrogate body.

This procedure is not without its difficulties. The key test for success is that the procedure results in healthy offspring. The offspring produced from the gametes in the study did indeed seem healthy and active; their primordial germ cells, however, were not. First-generation primordial germ cells tend to exhibit problems including “eggs that are fragile, misshapen and sometimes dislodged from the complex of cells that supports them. When fertilized, the eggs often divide into cells with three sets of chromosomes rather than the normal two, and the rate at which the artificial primordial germ cells successfully produce offspring is only one-third of the rate for normal in vitro fertilization (IVF). While the procedure did produce viable offspring, the offspring are far less likely to reproduce effectively.

Other research groups using this method found that the primordial germ cells made from stem cells maintain some of the latter’s epigenetic programming rather than resetting themselves like natural primordial germ cells do. This is a problem because epigenetic factors, chemical modifications that affect whether a gene is turned on or off, accumulate over an organism’s lifetime. It is important that these epigenetic factors are erased when forming an embryo. If they are not fully reset, as in natural systems, this may indicate that the produced cells are like primordial germ cells, but do not qualify as actual primordial germ cells because they do not function as expected. One of the motivations in studying primordial germ cells is to understand epigenetic factors during development, and this procedure adds a new layer to the available data.

Purpose of This Research

Hayashi and Saitou were surprised at the number of people that called them hoping for help with infertility. Their stated purpose for producing gametes from iPSCs and ESCs is research-oriented, not clinical. While the goal is eventually to apply the procedure to human cells, Saitou cautions against jumping into using it for reproductive purposes; the risks are significant and could span generations. He hopes to use this procedure to produce oocytes for experimental purposes, since oocytes are in short supply.

Additionally, Hayashi and Saitou want to study how epigenetics affects development. According to a review article in Development, of which Saitou is first author, there are two moments in which the epigenetic landscape resets itself: At the pre-implantation embryonic stage, and when primordial germ cells are formed. Epigenetics are factors within DNA that tell the genes what to do, impacting whether and how they are expressed. One such factor is a chemical group called a methyl group, which attaches to particular nucleotides; the Development article investigates DNA demethylation as a key potential mechanism of epigenetic reprogramming–an issue Hayashi and Saitou’s research both promises to shed light on.

Bioethics Issues

The ethics issue is one of caution and discernment. The technique has only been shown to work in mice so far and may not even be safe in mouse models, let alone humans, so rushing into applications aimed at creating new human beings would be irresponsible. With new technologies, particularly ones that deal with such delicate matters as development, it is important to exercise caution and discernment, because these decisions will affect the health and well-being of the offspring.

For example, in the mice models, many of the offspring of the artificially-created mice were infertile. This could mean that couples would not only be passing down any genetic fertility issues they may have had, but also generating entirely new genetic infertility issues for their offspring. Furthermore, iPSCs and ESCs often acquire various sorts of abnormalities during culture, and as stem-cell biologist Harry Moore points out in the Nature News article, “There could be potentially far-reaching, multi-generational consequences if something went wrong in a subtle way.” This technology could have significant consequences for prospective future human beings; these need to be investigated and considered before embracing it as a new reproductive technique.

Additionally, depending on the procedure used, certain ethical questions arise whether the gametes are made for reproductive use or solely for research purposes. Some may consider research on human gametes, let alone the formation of an embryo, ethically problematic. In the published procedure employing mice models, both a male and a female parent are still required to produce the embryos. The progenitor cells are synthesized in the lab and grown in the body, making the current procedure like a new form of IVF, so in human models this would still constitute human embryonic research. At the other end of the spectrum, if researchers successfully produced an embryo entirely from a single source–self-fertilization using sperm and oocytes made from the same individual’s cells–the new embryo would be similar to a clone, and the same questions regarding identity and individuality that arise regarding cloning would apply. And, in any potential use of the procedure, if the primordial germ cells are made from ESCs rather than iPSCs, the ESCs are harvested at the expense of an already existing embryo.

In summary, it is important not to over-hype the application prospects of early scientific findings. In addition to the problems that remain with the procedure employed in mice, the developmental process of mice is very different from that of human beings. Hayashi and Saitou see going from mice to humans as practically starting over with a whole new system, which means human models are long way off. Nevertheless, it is important to preemptively discuss and evaluate new scientific research with potentially substantial bioethical implications while it is still in the speculative stages, rather than trying to put the proverbial genie back into the bottle after the experiments have already been done.